You may find this an interesting read :-
http://www.telegraph.co.uk/money/main.jhtml?xm l=/money/2007/12/03/cmpen03.xml
Personally I think a mixture of both pensions and ISA's is best. Pensions give you a guaranteed but often small and taxable income in relation to the amount of money you have saved,especially if you retire early.
While ISA's enable you to take the money when you want to,are totally free from tax,thus giving you more potential to retire earlier.
Obviously everyone's circumstances are different and if you have access to a final salary pension through your employer then grab it with both hands.But let's say you want to retire early at 55 and you've got a personal pension pot of �100,000. You would get a pension of around �3,500 per year!!!
Source:-
http://www.annuity-bureau.co.uk/AF_CMS/Resourc es/Annuity%20bureau/rates_rpi.html
However if you have �100,000 in an ISA with an interest rate of 6% (current aprox rates),you could get an income of �6,000 per year and you still have the option of spending some capital aswell. Although the income will obviously fall as a result.
That's how I look at it anyway!